Radiation curable materials are used extensively in the production of optical fiber, particularly in the production of protective coatings that surround the optical fiber. One class of radiation curable materials can be converted from a liquid to a solid upon exposure to light energy in the ultraviolet (UV) spectrum, e.g., wavelengths ranging from about 200 to about 400 nanometers. Most UV radiation curable materials contain a monomer and oligomer mixture with an added photo-initiator package that facilitates cross-linking upon exposure to the radiation. The extent of the cross-linking reaction is dependent upon the amount of exposure to the radiation.
The coatings surrounding an optical fiber can be quickly cured by drawing the coated optical fiber past a high power UV curing lamp at a speed proportional to the radiation density provided by the curing lamp. However, it is known that atmospheric oxygen can react with certain coating materials to form peroxy radicals that can interfere with the curing process and may cause incomplete curing, particularly at the surface of the coating where the oxygen is present. Therefore, when a high degree of cure is required on the secondary or exterior coating layer, the coated fiber may be cured in an inert environment. For example, the coated fiber can be drawn through a center tube filled with inert gas (such as nitrogen) to maintain an oxygen-free atmosphere while the coating is cured. The center tube is usually made of UV transparent quartz to allow UV light to pass from the curing lamp to the fiber coatings with minimal attenuation, absorption, or scattering of the light.
During the normal UV curing process, temperatures within the UV curing tube can often reach or exceed 90 degrees Celsius due to the heat produced by the UV lamps, even with high flow rates of nitrogen. The elevated temperature can cause a portion of the uncured coating material to volatize (or off-gas) and, unfortunately, the volatiles can form deposits on the inner surface of the center tube. These coating deposits can cause significant clouding on the inner surface of the center tube and can severely attenuate the UV curing light to the point where the fiber coatings may not be sufficiently cured. Therefore, the center tubes are routinely replaced on a regular basis, often with each new preform.
A need remains for improved systems and methods for curing optical fiber coatings.